linux_aura: Default to system titlebars under some window managers.

This moves the existing window manager check in BrowserWindowGtk to
shared X11 code.

BUG=317859
R=mgiuca@chromium.org, msw@chromium.org, sadrul@chromium.org

Review URL: https://codereview.chromium.org/174223004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252586 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252586